A variety of combustion processes produce different classifications of nitrogen oxides (NO.sub.x). "Fuel NO" oxidation of nitrogen components contained in various fuels. "Prompt NO " results from NO promptly formed when hydrocarbon fuels such as fuel oil, kerosene, and LPG are burned at an air ratio (the ratio of the actual air supply to amount of air stoichiometrically required for the combustion of fuel) of about 0.5 to 1.4, permitting hydrocarbons to react with the nitrogen in the air and further to undergo several reactions. "Thermal NO" is produced when the nitrogen and oxygen in the air react at a high temperature in the course of combustion.
With the advent of contemporary environmental emission standards being imposed by various governmental authorities and agencies involving ever stricter regulations, methods and apparatus to suppress the formation of nitrogen oxides during combustion of hydrocarbon fuels with air are becoming increasingly numerous.
Previously known methods for reducing nitrogen oxide production include: (1) a method in which air is supplied in two stages to form a first-stage combustion zone having an air ratio of up to 1.0 and a second-stage combustion zone down-stream from the first-stage zone with a supplemental air supply; (2) a method which uses a combustion furnace equipped with a plurality of burners and in which air is supplied to each burner at an excessive or somewhat insufficient rate relative to the fuel supply to effect combustion is admixed with the fuel on the air for combustion by circulation; and (3) a method in which the exhaust gas resulting from combustion is admixed with the fuel or the air for combustion by circulation.
The first of these methods of reducing NO.sub.x is unable to suppress the formation of prompt NO when the air ratio of the first-stage combustion zone is in the usual range of 0.5 to 1.0. Even if it is attempted to inhibit the formation of prompt NO to the greatest possible extent as by maintaining the air ratio at about 0.5, the unburned components will react with the secondary air where it is supplied, giving prompt NO. Thus the method fails to produce the desired result. With the second method in which the fuel is burned at an air ratio (usually 0.6 to 1.4) at which each burner can burn the fuel independently of another, the formation of thermal NO and prompt NO inevitably results. The third method is not fully feasible since the exhaust, if circulated at an increased rate to effectively inhibit NO.sub.x, will impair steady combustion.
Other known methods have burned a fuel-lean mixture in a primary stage and fuel-rich in a secondary stage diluted with flue gas where the second stage is located radially around the primary stage as in U.S. Pat. No. 4,496,306 (the '306 patent). The '306 patent, however, does not teach premixing the first-stage mixture and does not teach diluting the second-stage mixture with steam or other inert fluids. Previous methods have also taught diluting with water a down stream radially located secondary stage as in Japanese Patent No. 52-74930. Dilution with steam is not taught in the secondary stage and premixing of the first stage is not taught. It would be advantageous to have a process of reducing nitrogen oxide formation which overcomes the deficiencies of previously known methods.